U.S. patent application number 11/551621 was filed with the patent office on 2007-03-01 for lithographic type microelectronic spring structures with improved contours.
This patent application is currently assigned to FormFactor, Inc.. Invention is credited to Benjamin N. Eldridge, Stuart W. Wenzel.
Application Number | 20070045874 11/551621 |
Document ID | / |
Family ID | 27002649 |
Filed Date | 2007-03-01 |
United States Patent
Application |
20070045874 |
Kind Code |
A1 |
Eldridge; Benjamin N. ; et
al. |
March 1, 2007 |
Lithographic Type Microelectronic Spring Structures with Improved
Contours
Abstract
Improved lithographic type microelectronic spring structures and
methods are disclosed, for providing improved tip height over a
substrate, an improved elastic range, increased strength and
reliability, and increased spring rates. The improved structures
are suitable for being formed from a single integrated layer (or
series of layers) deposited over a molded sacrificial substrate,
thus avoiding multiple stepped lithographic layers and reducing
manufacturing costs. In particular, lithographic structures that
are contoured in the z-direction are disclosed, for achieving the
foregoing improvements. For example, structures having a U-shaped
cross-section, a V-shaped cross-section, and/or one or more ribs
running along a length of the spring are disclosed. The present
invention additionally provides a lithographic type spring contact
that is corrugated to increase its effective length and elastic
range and to reduce its footprint over a substrate, and springs
which are contoured in plan view. The present invention further
provides combination (both series and parallel) electrical contacts
tips for lithographic type microelectronic spring structures. The
microelectronic spring structures according to the present
invention are particularly useful for making very fine pitch arrays
of electrical connectors for use with integrated circuits and other
substrate-mounted electronic devices, because their performance
characteristics are enhanced, while at the same time, they may be
manufactured at greatly reduced costs compared to other
lithographic type microelectronic spring structures.
Inventors: |
Eldridge; Benjamin N.;
(Danville, CA) ; Wenzel; Stuart W.; (San
Francisco, CA) |
Correspondence
Address: |
N. KENNETH BURRASTON;KIRTON & MCCONKIE
P.O. BOX 45120
SALT LAKE CITY
UT
84145-0120
US
|
Assignee: |
FormFactor, Inc.
|
Family ID: |
27002649 |
Appl. No.: |
11/551621 |
Filed: |
October 20, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09710539 |
Nov 9, 2000 |
|
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|
11551621 |
Oct 20, 2006 |
|
|
|
09364788 |
Jul 30, 1999 |
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09710539 |
Nov 9, 2000 |
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Current U.S.
Class: |
257/784 ;
257/E21.582; 257/E23.014; 257/E23.078 |
Current CPC
Class: |
H01L 24/72 20130101;
H01L 2924/01046 20130101; H01L 2924/30107 20130101; H01L 2924/14
20130101; H01L 2924/01082 20130101; H01L 2924/01074 20130101; H01L
2924/01013 20130101; H01L 2924/01079 20130101; H01L 2924/19041
20130101; H01L 2924/01023 20130101; G01R 3/00 20130101; H01L
2924/01078 20130101; H01L 2924/01019 20130101; H05K 7/1069
20130101; H01L 2924/01029 20130101; H01L 2924/01006 20130101; H01L
23/4822 20130101; H05K 3/4092 20130101; G01R 1/07342 20130101; G01R
1/06727 20130101; Y10T 29/4922 20150115; H01L 2924/01033 20130101;
G01R 1/06733 20130101; H01L 2924/01027 20130101; Y10T 29/49151
20150115; H01L 2924/01049 20130101; H01L 2924/01042 20130101; H01L
2924/01012 20130101; H01L 2924/01047 20130101 |
Class at
Publication: |
257/784 |
International
Class: |
H01L 23/52 20060101
H01L023/52 |
Claims
1-70. (canceled)
71. A method of forming a contact structure on a terminal of an
electronic component, said method comprising: forming a patterned
sacrificial material on said electronic component, said sacrificial
material patterned to include an opening over said terminal
defining a base of said contact structure and a molded surface
defining a beam of said contact structure, said molded surface
contoured to define a cross-sectional-width contour for said beam
to increase an area moment of inertia of said beam; forming said
contact structure in said opening and on said molded surface; and
removing said sacrificial material from said electronic
component.
72. The method of claim 71, wherein said step of forming said
contact structure comprises: depositing a seed material; and
depositing a contact structure material on said seed material.
73. The method of claim 72, wherein said step of depositing a
contact structure material comprises electroplating said contact
structure material on said seed material.
74. The method of claim 71, wherein said step of forming said
contact structure comprises: depositing a seed material over said
sacrificial material; forming a patterned masking material over
said seed material, said masking material patterned to have an
opening corresponding to said opening in said sacrificial material
and said molded surface of said sacrificial material; and
depositing a contact structure material on said seed material
exposed through said opening in said masking material.
75. The method of claim 74, wherein said step of depositing a
contact structure material comprises electroplating said contact
structure material on said seed material.
76. The method of claim 71, wherein said step of forming a
patterned sacrificial material comprises: depositing a layer of
sacrificial material on said electronic component; and stamping
said sacrificial material to form said opening and said molded
surface.
77. The method of claim 71, wherein said electronic component is a
semiconductor die.
78. The method of claim 77, wherein said semiconductor die is one
of a plurality of semiconductor dice composing an unsingulated
semiconductor wafer.
79. The method of claim 71 further comprising forming a plurality
of said contact structures on a plurality of terminals of said
electronic component, wherein said step of forming a patterned
sacrificial material on said electronic component comprises:
patterning said sacrificial material to include a plurality of
openings over said plurality of terminals, each opening defining a
base of one of said plurality of said contact structures, and
forming a plurality of molded surfaces, each defining a beam of one
of said contact structures, each said molded surface contoured to
define a cross-sectional width for said beam to increase an area
moment of inertia of said beam; said method further comprising
forming said plurality of contact structures each in one of said
openings and on one of said molded surfaces.
80. The method of claim 71, wherein said molded surface is further
contoured to define lengthwise contour for said beam.
81. The method of claim 80, wherein said lengthwise contour
comprises a compound curve.
82. The method of claim 80, wherein said lengthwise contour
comprises corrugations.
83. The method of claim 71, wherein said cross-sectional-width
contour is generally "V" shaped.
84. The method of claim 71, wherein said cross-sectional-width
contour is generally "U" shaped.
85. The method of claim 71, wherein said cross-sectional-width
contour comprises a rib.
86. The method of claim 85, wherein said cross-sectional-width
contour comprises a plurality of ribs.
87. The method of claim 71, wherein said beam, viewed in a
direction normal to a surface of said electronic component, is
generally triangular shaped.
88. The method of claim 71, wherein said beam, viewed in a
direction normal to a surface of said electronic component,
comprises a serpentine shape.
89. The method of claim 71, wherein said beam, viewed in a
direction normal to a surface of said electronic component,
comprises a "C" shape.
90. The method of claim 71, wherein said beam, viewed in a
direction normal to a surface of said electronic component,
comprises a "U" shape.
91. The method of claim 71, wherein said beam, viewed in a
direction normal to a surface of said electronic component,
comprises an "S" shape.
Description
RELATED APPLICATION
[0001] This application is a continuation-in-part of co-pending
application Ser. No. 09/364,788, filed Jul. 30, 1999, entitled
"INTERCONNECT ASSEMBLIES AND METHODS," by Eldridge and Mathieu,
which is incorporated by reference herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to electrical contact elements
for electrical devices, and more particularly to lithographic type,
microelectronic spring contacts with improved contours.
[0004] 2. Description of Related Art
[0005] Recent technological advances, such as described in U.S.
Pat. No. 5,917,707 to Khandros et al., have provided small,
flexible and resilient microelectronic spring contacts for mounting
directly to substrates, such as semiconductor chips. The '707
patent discloses microelectronic spring contacts that are made
using a wire bonding process that involves bonding a very fine wire
to a substrate, and subsequent electroplating of the wire to form a
resilient element. These microelectronic contacts have provided
substantial advantages in applications such as back-end wafer
processing, and particularly for use as contact structures for
probe cards, where they have replaced fine tungsten wires. It is
further recognized, as described, for example, in U.S. Pat. Nos.
6,032,356 and 5,983,493 to Eldridge et al, that such
substrate-mounted, microelectronic spring contacts can offer
substantial advantages for making electrical connections between
semiconductor devices in general, and in particular, for the
purpose of performing wafer-level test and burn-in processes.
Indeed, fine-pitch spring contacts offer potential advantages for
any application where arrays of reliable electronic connectors are
required, including for making both temporary and permanent
electrical connections in almost every type of electronic
device.
[0006] In practice, however, the cost of fabricating fine-pitch
spring contacts has limited their range of applicability to less
cost-sensitive applications. Much of the fabrication cost is
associated with manufacturing equipment and process time. Contacts
as described in the aforementioned patents are fabricated in a
serial process (i.e., one at a time) that can not be readily
converted into a parallel, many-at-a-time process. Thus, new types
of contact structures, referred to herein as lithographic type
microelectronic spring (or contact, or spring contact) structures,
have been developed, using lithographic manufacturing processes
that are well suited for producing multiple spring structures in
parallel, thereby greatly reducing the cost associated with each
contact. Exemplary lithographic type spring contacts, and processes
for making them, are described in the commonly owned, co-pending
U.S. patent applications "LITHOGRAPHICALLY DEFINED MICROELECTRONIC
CONTACT STRUCTURES, Ser. No. 09/032,473 filed Feb. 26, 1998 by
Pedersen and Khandros, and "MICROELECTRONIC CONTACT STRUCTURES",
Ser. No. 60/073,679, filed Feb. 4, 1998 by Pedersen and Khandros,
both of which are incorporated herein by reference.
[0007] Lithographic type microelectronic spring contacts are
subject to different design considerations than the plated and
bonded wire microelectronic contacts currently in use, because of
the characteristics of the lithographic manufacturing process. For
example, lithographic type contacts are typically much smaller than
wire-type microelectronic contacts, and tend to have characteristic
cross-sections of relatively low-aspect ratio (i.e., flat) shape,
in contrast to the circular or elliptical cross-sections typical
for wire contacts. Because of their typical structural shape,
lithographic type springs with essentially flat, rectangular
cross-sections typically have relatively low stiffness (spring
rates) and relatively small elastic ranges (that is, they may be
deflected for only a short distance before becoming permanently
deformed). Consequently, it is difficult to achieve the desired
contact force needed to make a reliable electrical contact at the
contact tip, without exceeding the elastic range of the spring, and
thereby potentially damaging it.
[0008] Additionally, lithographically-defined contacts typically
have a proportionally small "z-component," that is, they may extend
away from the substrate in a perpendicular ("z") direction
proportionally less than wire-type microelectronic contacts. This
also limits the elastic range of the spring, because the contact
force is typically applied in the z direction. One approach for
providing adequate z-extension, for example, as disclosed in the
above-referenced U.S. patent application Ser. No. 09/032,473 and
60/073,679, is to fabricate the spring structures using a series of
lithographic steps, thereby building up the z-component extension
with several lithographic layers. However, the use of multiple
layers adds undesirable cost and complexity to the manufacturing
process. Layered structures are also subject to undesirable stress
concentration and stress corrosion cracking, because of the
discontinuities (i.e., stepped structures) that result from
layering processes.
[0009] Microelectronic spring structures are preferably provided
with ample z-extension to permit mounting components, such as
capacitors, below the structure. Adequate z-extension, together
with adequate elastic range, is also desirable for reducing the
amount of vertical positioning precision needed to make an
electrical connection using the spring structure. Adequate
stiffness is desired to ensure that the tip of the contact is
applied to its electrical contact pad with sufficient force to
ensure that a reliable electrical connection is made. Finally,
improved strength and crack resistance is desirable for increasing
the reliability and service life of the spring structure. The
typical characteristics of very small feature size, relatively flat
rectangular cross-section, and less z-component extension in
proportion to spring length, make it very difficult to fabricate
lithographically defined spring structures with adequate strength,
stiffness, elastic range, and z-extension, to serve as reliable
microelectronic spring contact structures.
[0010] A need therefore exists for an improved, lithographic type,
microelectronic spring structure with improved spring
characteristics, such as improved strength, stiffness, resistance
to stress concentration cracking, and elastic range. A need further
exists for an improved lithographic type microelectronic spring
structure that can be fabricated in a single layer, thereby
eliminating process layering steps and the associated costs.
Furthermore, a need exists for an improved lithographic type
microelectronic spring structure with greater z-component
extension.
SUMMARY OF THE INVENTION
[0011] The present invention provides lithographically defined
spring structures and methods to address the foregoing
difficulties, while achieving adequate z-extension without
requiring the use of multiple stepped lithographic layers.
Lithographic type microelectronic spring contact structures are
provided that are triangular in shape in plan view, low-aspect
ratio rectangular in cross-section, and having z-component
extension along a linear or curved slope. In particular, spring
structures according to the present invention are suitable for
being fabricated by depositing a layer of integrated spring
material over a molded form, thereby greatly reducing the number of
processing steps required. At the same time, the spring structures
according to the present invention are contoured to provided
numerous performance improvements. For example, structures having a
U-shaped cross-section, a V-shaped cross-section, and/or a rib
running along a length of the spring are provided. Such contouring
provides spring structures with a higher stiffness and/or strength,
thereby providing a higher spring force at the contact tip and/or
working range for a given amount of spring material. The present
invention additionally provides a lithographic type spring contact
that is corrugated to increase its effective length and elastic
range. Contouring of the spring structure according to the present
invention further includes contouring in the plane of the
substrate, to provide increased elastic range within a decreased
footprint. The present invention further provides a variety of
improved tips for making electrical contacts with lithographic type
microelectronic spring structures.
[0012] A more complete understanding of the improved lithographic
type microelectronic spring structures will be afforded to those
skilled in the art, as well as a realization of additional
advantages and objects thereof, by a consideration of the following
detailed description of the preferred embodiment. Reference will be
made to the appended sheets of drawings which will first be
described briefly.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1A is a perspective view of an exemplary contoured
lithographic type microelectronic spring structure according to the
present invention, having a V-shaped cross-section.
[0014] FIG. 1B is a cross-sectional view of the spring structure
shown in FIG. 1A, taken along the line indicated by arrows 1B in
FIG. 1A, additionally showing a stop structure disposed over the
base of the spring.
[0015] FIG. 2 is a diagram for illustrating various general
concepts and characteristics applicable to spring structures.
[0016] FIG. 3 is a side cross-sectional view of an exemplary spring
structure according to the present invention while used for making
an electrical connection between two substrates.
[0017] FIG. 4A is a side cross-sectional view of a process
structure during an exemplary initial step of a process for making
a lithographic type microelectronic spring structure according to
the present invention.
[0018] FIG. 4B similarly shows a process structure during an
exemplary step following the step shown in FIG. 4A.
[0019] FIG. 4C similarly shows a process structure during an
exemplary step following the step shown in FIG. 4B.
[0020] FIG. 4D similarly shows a process structure during an
exemplary step following the step shown in FIG. 4C.
[0021] FIG. 4E similarly shows a process structure during an
exemplary step following the step shown in FIG. 4D.
[0022] FIG. 4F similarly shows a process structure during an
exemplary step following the step shown in FIG. 4E.
[0023] FIG. 4G similarly shows a process structure during an
exemplary step following the step shown in FIG. 4F.
[0024] FIG. 4H similarly shows a process structure and resulting
spring structures during an exemplary step following the step shown
in FIG. 4G.
[0025] FIG. 5A shows a cross-sectional view, taken along the line
indicated by arrows 5A in FIG. 1A, of an exemplary V-shaped spring
structure according to the present invention.
[0026] FIG. 5B shows a cross-section of an exemplary U-shaped
spring structure according to the present invention, viewed
similarly to FIG. 5A.
[0027] FIG. 5C shows a cross-section of an exemplary spring
structure with a flat rectangular cross-section, viewed similarly
to FIG. 5A.
[0028] FIG. 5D shows a cross-section of an exemplary ribbed spring
structure according to the present invention, viewed similarly to
FIG. 5A.
[0029] FIG. 6 is a perspective view of an exemplary contoured
lithographic type microelectronic spring structure according to the
present invention, having a U-shaped cross-section.
[0030] FIG. 7A is a perspective view of an exemplary contoured
lithographic type microelectronic spring structure according to the
present invention, having a longitudinal rib extending above the
spring.
[0031] FIG. 7B is a cross-sectional view of the spring structure
shown in FIG. 7A, taken along the line indicated by arrows 7B.
[0032] FIG. 8 is a perspective view of another exemplary contoured
lithographic type microelectronic spring structure according to the
present invention, having a longitudinal rib extending below the
spring.
[0033] FIG. 9A is a perspective view of an exemplary contoured
lithographic type microelectronic spring structure according to the
present invention, having longitudinal corrugations.
[0034] FIG. 9B is a cross-sectional view of the spring structure
shown in FIG. 9A, taken along the line indicated by arrows 9B.
[0035] FIG. 11A is a perspective view of a contoured spring having
a plurality of longitudinal ribs of tapering thickness.
[0036] FIGS. 11B and 11C are cross-sectional views of the contoured
spring shown in FIG. 11A.
[0037] FIG. 10 is a perspective view of an exemplary contoured
lithographic type microelectronic spring structure according to the
present invention, having a longitudinal rib extending above the
spring, and longitudinal corrugations.
[0038] FIG. 12A is a perspective view of an x-y folded design for a
spring structure, having a split beam.
[0039] FIG. 12B is a plan view of the spring structure of FIG.
12A.
[0040] FIG. 13A is a perspective view of an alternative x-y folded
design for a spring structure, without a split beam.
[0041] FIG. 12B is a plan view of the spring structure of FIG.
13A.
[0042] FIG. 14A is a perspective view of an alternative x-y folded
design for a spring structure, having a serpentine beam.
[0043] FIG. 14B is a plan view of the spring structure of FIG.
14A.
[0044] FIG. 15 is a perspective view of a spring structure for
connecting to multiple contact pads, for providing split inputs for
tuned devices.
[0045] FIG. 16A is a perspective view of an exemplary pointed
contact tip of a spring structure.
[0046] FIG. 16B is a perspective view of an exemplary spherical
contact tip of a spring structure.
[0047] FIG. 16C is a perspective view of an exemplary pyramidal
contact tip of a spring structure.
[0048] FIG. 16D is a perspective view of an exemplary parallel
combination contact tip of a spring structure, including pointed,
spherical, and pyramidal contact tips.
[0049] FIG. 16E is a perspective view of an exemplary serial
combination contact tip of a spring structure, having both
pyramidal and spherical aspects.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0050] The present invention satisfies the need for microelectronic
spring structure, that overcomes the limitations of prior art
spring contacts. In the detailed description that follows, like
element numerals are used to describe like elements illustrated in
one or more figures. Various terms and acronyms are used throughout
the detailed description, including the following:
[0051] "Area moment of inertia," sometimes denoted as "I," is a
section property defined as the integral of the section area times
the distance from the neutral bending axis squared, that is:
.intg.z.sup.2dA where z is the distance from the neutral axis of
bending and dA is an element of the area being integrated. For
example, for a rectangular cross-section having a width "w", a
thickness "t," and a neutral axis through its centroid (1/2t) on
the y-axis at z=0, the area moment of inertia is wt.sup.3/12.
[0052] "Contour," when used as a verb, means to form a curved
surface. When used as a noun, "contour" means a curved line defined
by the intersection of a curved surface and a plane. A "contoured
surface" means a formed, curved surface.
[0053] "Elastic deflection ratio" means the dimensionless ratio of
the elastic range (placed in the numerator of the ratio) to the
projected length of the undeflected spring in the same direction
(placed in the denominator). For example, with respect to a
vertical downward force applied to a cantilevered beam spring, the
elastic deflection ration is the vertical elastic range "c" divided
by the z-extension of the spring "h." Being dimensionless, it may
be expressed as a percentage; for example, 10% is equivalent to a
ratio of 0.1.
[0054] "Elastic range" means the maximum distance that a spring may
be deflected in the direction of an applied force, before the
spring is permanently deformed (which occurs when the yield
strength of the spring is exceeded).
[0055] "Lithographic" refers to manufacturing processes used for
making semiconductor devices by printing (or otherwise applying)
thin layers of various materials in patterns on a substrate.
[0056] "Lithographic type" means having a shape suitable for making
by a lithographic process. More precisely, a "lithographic type
microelectronic spring structure" is one having a contoured sheet
shape that is capable of being defined by deposition of a patterned
material layer (typically, a layer of uniform thickness) on a
substrate. Lithographic type contact structures are generally well
suited for being fabricated at the same scale as the integrated
circuits. For example, a lithographic type contact typically has a
beam with its smallest dimension (thickness) not greater than about
50 microns, and more typically about 0.04 to 20 microns. The
dimensional limit reflects a practical, rather than an inherent
limit, of the lithographic process, and it is contemplated that
lithographic type contact structures may be fabricated at larger
scales for some applications.
[0057] "Spring rate" means a measure of spring stiffness, commonly
expressed as k=F/.delta., where k is the spring rate, F is the
magnitude of a force applied to a working end of a spring, and
.delta. is the distance that the spring is deflected in the
direction of force F.
[0058] The foregoing definitions are not intended to limit the
scope of the present invention, but rather are intended to clarify
terms that are well understood by persons having ordinary skill in
the art, and to introduce new terms helpful for describing the
present invention. It should be appreciated that the defined terms
may also have other meanings to such persons having ordinary skill
in the art. These and other terms are used in the detailed
description below.
[0059] Contact structures according to the present invention are
particularly well-suited to making electrical connections to
microelectronic devices having contact pads disposed at a
fine-pitch, or where a large array of economical microelectronic
spring contacts is desired. As used herein, the term "fine-pitch"
refers to microelectronic devices that have their contact pads
disposed at a spacing of less than about 130 microns (5 mils), such
as 65 microns (2.5 mils). However, structures of the present
invention may also be used in coarser-pitch applications, if
desired. The advantages of the present invention are realized in
part from the close tolerances and small sizes that can be realized
by using lithographic rather than mechanical techniques to
fabricate the contact elements. However, the use of lithographic
techniques does not, by itself, result in effective and reliable
contact structures disposed at a fine-pitch. Lithographic type,
microelectronic spring contacts according to the structures and
forms disclosed herein provide a greatly increased probability of
success, and a greatly extended range of applications, compared to
lithographic type springs having essentially linear beams with
rectangular cross-sections.
[0060] Resilient contact structures, as known in the art, are
subject to particular performance requirements, which vary in
degree between applications. These requirements typically relate to
contact force, wipe, clearance, contact angle, elastic range,
z-extension, repeatability, and longevity. The contact structures
according to the present invention provide advantages for each of
the foregoing performance areas, as is made apparent by the
description that follows. An exemplary microelectronic spring
structure for providing such advantages is shown in FIGS. 1A and
1B. The cross-section shown in FIG. 1B is taken along the line
indicated by arrows 1B in FIG. 1A. As indicated by arrows 31 and as
used herein, the direction normal to substrate 23 is the z-axis
direction; the direction parallel to the projected length of beam
30 onto substrate 23 is the x-axis direction; and the y-axis
direction is normal to the plane defined by the z-axis and
x-axis.
[0061] The microelectronic spring structure 22 of FIGS. 1A and 1B
comprises a base 28 and a beam 30 integrally formed from at least
one layer of electrically conductive, resilient material. Substrate
23 is typically a semiconductor substrate for an integrated circuit
having numerous electrical terminals, one of which is shown as the
contact pad 33 in FIG. 1A. Contact pads, such as contact pad 33,
are typically coupled by conductive traces, such as trace 35, to
internal circuitry within the integrated circuit. As known in the
semiconductor art, substrate 23 is typically comprised of numerous
layers, such as insulating layers interposed with conducting and
semiconducting layers, and a passivating layer optionally provided
on the top surface 55 of the substrate 23. The passivating layer
may be an insulating layer or a polysilicon layer or other layers
as known in the art. In some embodiments of the invention, a
contact pad 33 is electrically and mechanically coupled to an
intermediate conducting layer 32 which is disposed above it, as
shown in FIG. 1A. When present, intermediate layer 32 is typically
a manufacturing artifact of a shorting layer used during an
electroplating step of a process for forming the microelectronic
spring structure. A stop structure 25, as further described in the
co-pending application Ser. No. 09/364,855, filed Jul. 30, 1999,
entitled "INTERCONNECT ASSEMBLIES AND METHODS," by Eldridge and
Mathieu, which is hereby incorporated herein by reference, is
optionally provided to prevent over-compression of spring structure
22 by force "F." Spring structure 22 provides for conduction of
electrical signals and/or power between a tip 26 of beam 30,
through the beam 30 of resilient material, intermediate layer 32,
and contact pad 33, and finally through conductive trace 35 to an
integrated circuit in substrate 23. It should be appreciated that
the microelectronic spring structure 22 of FIGS. 1A and 1B may also
be used for other types of interconnect assemblies, such as probe
card assemblies, interposers, and other connection systems where
electrical contact to or through a substrate is desired.
[0062] Microelectronic contact structures according to the present
invention are typically configured as a cantilevered beam, having a
fixed base and a free tip, as described above. This basic geometry
is preferred for lithographic type, microelectronic spring contact
structures according to the invention for several reasons.
Lithographic manufacturing processes, unlike alternative spring
manufacturing processes such as wire forming, are best suited for
making shapes that may be defined by projection onto a surface.
Such shapes are capable of being formed in a single step of a
lithographic process. In contrast, certain three dimensional
curves, such as coils or helixes commonly found in wire-formed
springs, cross over themselves and cannot be defined by projection
onto a surface. Such shapes therefore require multiple lithographic
steps to assemble, and cannot be made lithographically as a single,
integrally formed piece. Coiled shapes are further undesirable
because of electrical induction. Therefore, the cantilevered beam
configuration is desirable for lithographic springs because fewer
process steps are required, and the spring structure can be formed
from an integrated mass of material. Also, the cantilevered beam,
with a single attached base and a tip on the opposite end of a
spring beam, is more nearly statically determinate, and thus is
more easily modeled and tested than more complex configurations,
such as springs with more than one base. Additionally, the
cantilevered beam is capable of resilient motion in at least two
dimensions, thereby providing both wipe (desirable for making an
electrical connection) and z-deflection, for compensating for
misalignment between substrates, and for providing the spring force
needed for maintaining an electrical connection.
[0063] Basic characteristics of linear cantilevered spring beams
are illustrated by FIG. 2. Undeflected beam 51, of length "L," is
fixed at x-axis 41 (representing a substrate surface). Beam 51
extends both vertically in the direction of z-axis 43, and
horizontally along the substrate for a projected horizontal
distance "L.sub.p." A free tip of the beam is located a distance
"h" from the substrate surface. When a vertical force "F" is
applied to the free tip of beam (as by an approaching parallel
substrate), the beam is deflected to the position indicated by
dashed lines 53. Thus, the free tip of the beam moves a vertical
distance "c" and a horizontal distance "d" (wipe). At the point at
which further deflection will cause the beam material to yield and
plastically deform (that is, at the elastic limit of beam 51), the
vertical deflection "c" represents the elastic range relative to a
vertical force "F." Thus, at the elastic limit, the ratio c/h is
the elastic deflection ratio, beyond which deflection is inelastic.
In a beam of constant cross-section, the point of maximum stress
under application of force "F" is at the attached end of the beam,
and the stress gradually lessens to a minimum value at the free
tip.
[0064] Beam 51 is preferably tapered from a relatively wide width
at its fixed base to a relatively narrow width at its tip, to
compensate for stress distribution in the cantilevered beam. Such
triangular-shaped beams are relatively more structurally
"efficient" (capable of bearing a higher tip force "F" for a beam
of given mass) than rectangular shaped beams of the same
cross-sectional shape. However, a triangular shape may be less
electrically efficient, because its current-carrying capacity is
constrained by the relatively small beam cross-section at the tip
of a triangular beam. Thus, for certain applications, beams of
constant cross-section may be preferred. Spring beams according to
the present invention may thus be tapered, and/or be provided with
constant cross-sectional areas, depending on the requirements of
the application.
[0065] In addition, Spring beams according to the present invention
are preferably contoured across their width and along their length.
Contouring along the length of the beam provides a more favorable
deflected shape of the beam for purposes of making an electrical
connection between two substrates, as described below with
reference to FIG. 3. Contouring across the width of the beam
provides cross-sectional shapes having higher area moments of
inertia, compared to beams of the same mass having solid
rectangular cross-sections. As is well understood in the structural
arts, the stiffness of a beam of a given mass per unit length can
be dramatically improved by altering its cross-sectional shape. For
example, a box beam is much stiffer than a solid rectangular rod
having the same mass per unit length. Heretofore, it has not been
feasible to provide lithographic type contact structures with beams
having higher area moments of inertia than provided by solid
rectangular cross-sections. Also, there has been little or no
motivation to reduce beam masses, because the cost of beam material
for lithographic type beams is not significant. However, according
to the present invention, it is highly desirable to reduce the mass
of beam material in order to reduce the fabrication time required
and the area occupied in a top down view of the structure, which
determines the packing ability or minimum pitch at which the
springs may be mounted to a surface.
[0066] The theories and mathematical tools for predicting the
structural properties of a contoured spring structure are well
known in the art. Computational tools, such as finite element
methods, further make it possible to refine and optimize the shape
of complex spring structures under complex loading conditions.
Thus, using the contoured shapes according to the present
invention, it is now possible to construct microelectronic spring
structures using lithographic techniques that have a much wider
range of performance properties than heretofore possible. In
particular, the area moment of inertia, and thus the spring rate,
can be greatly increased by contouring the beam across its width.
Additionally, the spring shape can also be optimized to reduce
stress concentration, resulting in more efficient use of material.
Width-contoured springs can thus be made with much less material
than required for flat cross-section microelectronic spring
structures of a comparable spring rate and strength.
[0067] Reducing the amount of material required permits increased
processing throughput by decreasing the time required for
depositing a material. In addition, thin-layer deposition
techniques that were previously considered too slow or costly for
thick layers may be viable. For example, a flat spring design might
require a material thickness of 25 microns (about 1 mil) to achieve
a desired spring rate. Material of this thickness is typically
deposited by electroplating, which is known for high throughput and
cost-effectiveness in layers of this thickness. In contrast, a
spring contoured according to the present invention can achieve the
same spring rate using less material, for example, with only 5
microns (0.2 mils) of material thickness. If electroplating is
used, the processing throughput would be about 5 times higher for
the contoured spring. Additionally, CVD (chemical vapor deposition)
and PVD (physical vapor deposition), typically limited to
depositing layers up to about 5 microns thick, become viable
alternative deposition methods for the spring metal.
[0068] Referring again to FIGS. 1A and 1B, contouring of spring
beam 30 is visible in both views. Beam 30 is preferably contoured
across its width, as is shown in FIG. 1B illustrating a V-shaped
beam. An exemplary V-shaped cross-section of beam 30, having a
constant thickness "t," is shown in more detail in FIG. 5A. It will
be understood that an approximately constant thickness is a typical
result of lithographic type deposition processes, such as
electroplating, CVD, and PVD. From a comparison of the V-shape 50A
shown in FIG. 5A to the flat rectangular cross-section 50C of
equivalent thickness shown in FIG. 5C, it is evident that the
V-shaped cross-section 50A has a substantially greater area moment
of inertia, because the extension "a" of the shape across the
neutral axis 57 is much greater. Thus, a V-shaped cross-section may
be provided wherever the beam 30 is desired to be stiffened. In the
case of beam 30 shown in FIG. 1A, the V-shape is provided along the
entire length of beam 30. If desired, however, the contoured
cross-sectional shape may be provided along only a portion of the
beam length, or may be altered to provide varying beam stiffness
along the length of beam 30. This may be desirable where a portion
of the beam, for example, a tip portion, is designed to be
relatively flexible compared to a different portion, for example, a
base portion.
[0069] Additionally, beam 30 is preferably contoured lengthwise, as
is illustrated by the curve 34, representing a curved line at a
mid-plane of beam 30, visible in FIG. 1B. Beam 30 has a projected
length "L.sub.p" on substrate 23, which is less than an integrated
beam length "L" from the juncture 27 with base 28 to tip 26. Tip 26
is located a vertical distance "h" from substrate 23 when the
spring structure 22 is undeflected, and a distance "c" from the
upper surface of stop structure 25. In an embodiment of the
invention, beam 30 is preferably convex (as viewed from a viewpoint
above spring structure 22 looking towards substrate 23) at a lower
portion of the beam near base 28, and concave at an upper portion
near tip 26. Thus, curve 30 is preferably a compound curve, and the
entire beam 30 preferably follows approximately the same contour as
curve 34. Advantages of the compound curve 34 are illustrated in
FIG. 3, by way of comparison to a straight or convex beam, and to a
concave beam. FIG. 3 shows a spring structure in compression
between parallel substrates 23 and 23', for making electrical
contact between contact pads 33 and 33'. Structure 22 is attached
to substrate 23 at pad 33, and makes resilient contact with pad 33'
on substrate 23. A typical position of a straight or convex beam
under such compression between parallel substrates is shown by the
dashed outline 37. When deflected by approaching parallel
substrates 23 and 23', the straight or convex beam takes on an
increasingly convex shape with a slope approaching that of surface
55' of substrate 23'. Thus, the tip contact tends to be spread out
over a larger, less well-defined area 45, resulting in lower
contact pressure, and a poorer electrical connection. Conversely, a
concave beam, indicated by dashed outline 39, tends to approach the
substrate 23 near the base of the spring structure 22 when under
compression between parallel substrates. Under sufficient
compression, a concave beam will contact substrate 23 in the
vicinity of area 47, which may cause an electrical short circuit,
an undesirable change in spring rate, and/or stress concentration
in beam 30.
[0070] In contrast, the compound curve 34 of the present invention
avoids these disadvantages of linear, convex, and concave beams in
microelectronic contact applications. The concave upper portion 38
maintains the tip 26 of beam 30 inclined more perpendicularly
relative to surface 55', thereby maintaining a well-defined contact
area, high contact pressure, and good electrical contact with
contact pad 33'. The convex lower portion 36 of beam 30 resists
making contact with surface 55 of substrate 23, and tends to
maintain a relatively constant minimum distance (step height "s")
between beam 30 and substrate 23. The relatively constant step
height "s," in addition to preventing short circuits, is useful for
providing space for decoupling capacitors (not shown) having a
height less than "s." Such decoupling capacitors can be mounted to
either substrate 55 or 55'. In an embodiment of the invention, step
height "s" is preferably about 10% of the total z-extension
"h."
[0071] It should be noted that, for some applications, it is
preferable to fabricate a separate tip structure and mount it to
the tip portion 26 of beam 30. Separate tip structures are
discussed below with reference to FIGS. 16A-16E. Separate tip
structures have the disadvantage of requiring additional process
steps to fabricate. When a separate tip structure is needed despite
the added costs, the tip portion 26 of beam 30 may be formed with
its upper surface roughly parallel to the surface of the mating
substrate (as with a convex or essentially linear beam). However,
even if a separate tip structure is to be used, it is still
preferable that the upper portion 38 be mostly concave, to ensure
that the beam does not contact the mating substrate when the tip is
engaged in a contact pad of the mating substrate.
[0072] Prior to the present invention, it was not feasible to make
lithographic type spring structures with the necessary degree of
control over the shape of the spring contours for fabricating
contoured spring structures. The present invention, however,
provides a molding technique for fabricating contoured spring
structures. An adaptation of this molding technique for making
contoured spring structures according to the present invention is
illustrated in FIGS. 4A-4H. The fabrication of a single contact
structure will be described as exemplary of fabricating a plurality
of such contact structures, preferably all at the same time on the
same component. Typically, each of the contact structures
fabricated on a single component will be substantially identical to
one another. In the alternative, the dimensions and shape of each
contact structure can individually be controlled and determined by
the designer for given application requirements, and fabricated
using the method described herein as will be apparent to one
skilled in the art.
[0073] Referring to FIG. 4A, in a preparatory step of a method for
making a contoured spring, a substrate 23, optionally provided with
a contact pads 33 for connecting to a integrated circuit, is coated
with a moldable sacrificial layer 42. Sacrificial layer 42 may be
any number of materials, such as PMMA (poly methyl methacrylate),
which can be coated on a substrate to the desired thickness, which
will deform when pressed with a mold or stamp, which will receive
the resilient material to be deposited thereon, and which can then
readily be removed without damaging the spring structures 22.
Additional candidate materials for layer 42 include acrylic
polymers, polycarbonate, polyurethane, ABS plastic, various
photo-resist resins such as Novolac resins, epoxies and waxes. The
sacrificial layer 42 preferably has a uniform thickness slightly
greater than the desired z-extension "h" of the finished spring
structures. For example, if the desired z-extension is 50 microns
(about 2 mils), layer 42 may have a thickness of 55 microns (2.2
mils). Various methods known in the art, for example, spin coating
and lamination, may be used to deposit layer 42 onto substrate
23.
[0074] Also, a stamping tool 40, having a molding face provided
with different molding regions 44, 46, and 48, is prepared for
molding sacrificial layer 42. Various methods may be used to
prepare tool 40. For example, the stamping tool 40 may be formed
from a relatively hard material using a computer controlled, laser
ablation (micropulse) process as is known in the art.
[0075] Maximally protruding molding regions, or "teeth" 44 of tool
40 are used for deforming the sacrificial layer 42 in the area of
the contact pads 33, where the bases 28 of contact structures 22
will be formed. Contoured molding regions 46 are used for deforming
layer 42 where the contoured beams 30 of contact structures 22 will
be formed. In FIG. 4A, a contoured region for making a V-shaped
beam is shown in lengthwise cross-section. Maximally recessed
molding regions 48 are used for receiving excess material, i.e.,
"flash," pushed aside by molding regions 44 and 46. Molding regions
48 also define spacing between adjacent spring structures 22 on
substrate 23. Depending on the choice of materials for sacrificial
layer 42 and stamping tool 40, a layer of mold release material
(not shown) is optionally provided on the molding face of tool 40.
It should be recognized that further layers and material may be
present on substrate 23 without departing from the method described
herein. For example, a metallic shorting layer (not shown) is
optionally provided between layer 42 and substrate 23, to protect
any integrated circuits embedded in the substrate during processing
operations.
[0076] In a molding step illustrated in FIG. 4B, the stamping tool
40 is applied against substrate 23 with sufficient pressure to
bring the teeth 44 nearly to the surface of substrate 23, and to
fully mold layer 42 in all contoured molding regions 46. To avoid
damaging substrate 23, teeth 44 are preferably not brought into
contact with substrate 23. In a preferred embodiment, when teeth 44
have sunk into layer 42 to the desired depth, flash substantially
fills the maximally recessed regions 48 to form a surface
sufficiently uniform to permit later deposition of a layer of
masking material between the spring structures after the stamping
tool 40 is removed from layer 42. Stamping tool 40 may be heated to
assist deformation of layer 42, and then cooled to harden layer 42
into place. In an alternative embodiment, layer 42 is selected of a
material that is sufficiently deformable to flow under pressure
without application of heat, and sufficiently viscous to hold its
shape after tool 42 is removed. In yet another alternative
embodiment, heat, UV light, or chemical catalysts are used to
harden sacrificial layer 42 while under stamping tool 42, and then
tool 42 is removed. Whatever molding technique is used, the cycle
times are preferably relatively short to permit faster
manufacturing throughput.
[0077] FIG. 4C shows the shape of the sacrificial layer 42 after
removal of the stamping tool 42. A thin layer of residue 52 is
present over the area of each contact pad 33. Negative mold
surfaces 70 are also present, each bearing a negative impression of
the desired contour for the contoured beams to be formed therein.
It is necessary to remove the residue 52 in order to expose the
substrate 23 in the areas where the bases 28 of the contact
structures 22 will be formed. To remove the residue 52, the entire
substrate with its molded layer 42 may be isotropically etched by
immersion in a bath of etchants, by oxygen plasma, or other method
as known in the art. Isotropic etching is suitable for relatively
flat substrates for which the residue layer 52 is of a uniform
thickness in all places where the spring bases 28 will be formed.
Preferably, the isotropic etch is performed so as to remove the
residue 52 while at the same time reducing the thickness of layer
42 to equal the desired z-extension of the finished spring
structures 22. In the alternative, an anisotropic etching method
that etches more rapidly in the z-direction, such as reactive ion
etching, may be used. A z-anisotropic etch is preferably used in
cases where the substrate is relatively uneven, causing
non-uniformity in the thickness of residue 52.
[0078] The appearance of the molded sacrificial layer 42 after
etching is shown in FIG. 4D. The contact pads 33 are preferably
exposed, along with a surrounding area of substrate 23 sufficient
for providing adhesion of the base 28. In typical semiconductor
applications, an exposed area of substrate 23 of between about
10,000 and about 40,000 square microns, most preferably in excess
of about 30,000 square microns, is provided. After etching, the
mold surfaces 70 preferably take on the desired contoured shape,
and the distal tips of all mold surfaces 70 on substrate 23 are
preferably within essentially the same plane.
[0079] FIG. 4E shows substrate 23 after application of a seed layer
54 and a photo-resist layer 56. Seed layer 54 is typically a
relatively thin layer, such as about 4500 .ANG. (Angstroms; or
about 0.45 microns) thick, of sputtered metal for electroplating
the resilient spring material. In the alternative, surface
modifications of layer 42, e.g. plasma treatment, may be used to
render it conductive, thereby creating seed layer 54. In FIGS.
4E-4H, the relative thickness of seed layer 54 is greatly
exaggerated. Photo-resist layer 56 may be selected from various
commercially available resist materials, such as an
electrodeposited resist, Novolac liquid resists, or a
negative-acting dry film photo-resist. Photo-resist layer 56 is
cured in an appropriate manner, for example by exposing the layer
56 to UV light through a mask, except where the spring structures
22 are to be formed. The uncured portions of photo-resist layer 56
are then dissolved away by a suitable solvent, as known in the
art.
[0080] After the uncured portions of resist layer 56 are dissolved
away, exposed areas 59 of seed layer are revealed, as shown in FIG.
4G. Exposed areas 59 have the projected shape of the desired
microelectronic spring structure. For example, if a triangular beam
is desired, the exposed area has a generally triangular shape, in
plan view. One or more layers of resilient material may then be
electroplated or otherwise deposited onto the seed layer in the
exposed areas 59, using various methods as known in the art. Where
the seed layer is covered by resist layer 56, no electroplating
will occur. In the alternative, a layer of resilient material may
be built up using a process such as CVD or PVD selectively applied
to areas 59 through a mask, eliminating the need for seed layer 54.
Thus, using any of various deposition methods, a spring structure
comprising an integrally formed base and beam is formed on the
exposed area 59, as shown in FIG. 4G. The cured resist layer 56,
sacrificial material 42, and any residual seed layer 54, are then
dissolved away using a suitable etchant that is relatively slow to
etch the substrate 23 and the resilient material, as known in the
art. Freestanding spring structures 22, as shown in FIG. 4H, are
the result.
[0081] Suitable materials for the resilient material include but
are not limited to: nickel, and its alloys; copper, cobalt, iron,
and their alloys; gold (especially hard gold) and silver, both of
which exhibit excellent current-carrying capabilities and good
contact resistivity characteristics; elements of the platinum
group; noble metals; semi-noble metals and their alloys,
particularly elements of the palladium group and their alloys; and
tungsten, molybdenum and other refractory metals and their alloys.
Use of nickel and nickel alloys is particularly preferred. In cases
where a solder-like finish is desired, tin, lead, bismuth, indium,
gallium and their alloys can also be used. The resilient material
may further be comprised of more than one layer. For example, the
resilient material may be comprised of two metal layers, wherein a
first metal layer, such as nickel or an alloy thereof, is selected
for its resiliency properties and a second metal layer, such as
gold, is selected for its electrical conductivity properties.
Additionally, layers of conductive and insulating materials may be
deposited to form transmission line-like structures.
[0082] It should be recognized that numerous variations of the
above-described sequence of steps will become apparent to one
skilled in the art, for producing integrally formed spring
structures according to the present invention. For example, a
spring contact structure may be fabricated at an area on a
substrate which is remote from a contact pad to which it is
electrically connected. Generally, the spring contact structure may
be mounted to a conductive line that extends from a contact pad of
the substrate to a remote position. In this manner, a plurality of
spring contact structures can be mounted to the substrate so that
their tips are disposed in a pattern and at positions which are not
dependent on the pattern of the contact pads on the substrate.
[0083] However, although various adaptations may be made to the
method disclosed herein, in general, a molding or other forming
process using a relatively thick layer of sacrificial material,
such as layer 42, is preferred for providing adequate z-extension
without requiring building up of multiple layers of photo-resist.
Additionally, use of a deformable sacrificial material provides for
duplication and mass production of relatively complex, contoured
beam shapes.
[0084] Accordingly, in the preferred embodiments, the entire spring
structure (with the exception of optional features such as separate
tips) is definable in a layer of material deposited (such as by
electroplating, CVD, or PVD) on the surface of a mold form. The
resulting spring structures are thus comprised of an integral
sheet, which may comprise a single layer, or multiple coextensive
layers, of resilient, conductive, and/or resistive material. The
integrated sheet may be folded and contoured, and is preferably
essentially free of any overlapping portion in the direction that
the materials are deposited (typically from above the structure
towards a substrate), so it may be more readily formed by
depositing a layer or layers of material in a single open mold,
according to the process described above. A relatively small amount
of overlap, particularly when the overlapping portions are well
separated, may be achieved using some deposition methods, such as
electroplating in conjunction with a "robber" to drive electrically
charged material under an overhang.
[0085] The open molding process according to the present invention
may be adapted to form contoured beams for spring structures in a
wide variety of shapes and sizes. For the purpose of
microelectronic spring contact structures, certain sizes and
structural properties are preferred. A second, suitable range of
sizes and relative properties, outside of the preferred range, may
be readily achieved on electronic substrates using an open molding
process according to the invention, but is less useful for typical
electronic connection applications. A table of preferred and
outside ranges is provided below in Table 1. It should be
understood that the ranges stated in Table 1 should be helpful for
constructing suitable microelectronic spring structures according
to the invention, but should not be construed as strictly limiting
the invention. Microelectronic spring contact structures with one
or more dimensions or properties outside of a stated range may be
constructed, without departing from the scope of the invention
described herein. The symbols used in Table 1 are as presented in
FIGS. 2 and 3. TABLE-US-00001 TABLE 1 Preferred Symbol Description
Range Suitable Range L Beam length, integrated from 500-1000 .mu.m
15-10.sup.4 .mu.m base to beam tip (20-40 mils) (0.5-400 mils) h
Height of beam tip over 100-300 .mu.m 10-7000 .mu.m beam base (4-12
mils) (0.4-300 mils) t Beam thickness 10-30 .mu.m 0.1-500 .mu.m
(0.4-1.2 mils) (.04-20 mils) w Beam width (at widest point) 130-500
.mu.m 5-2500 .mu.m (5-20 mils) (0.2-100 mils) .alpha. Tip angle,
relative to a normal 20-70.degree. 0-180.degree. vector of a mating
substrate .beta. Base angle, relative to the 30-80.degree.
0-90.degree. mounting substrate surface k Spring rate (vertical)
30-250 mg/.mu.m 5-600 mg/.mu.m (1-6 g/mil) (0.1-15 g/mil) t/L Ratio
of beam thickness to .02-.06 .01-0.4 length c/h Elastic deflection
ratio 0.4-1.0 0.05-2.0 L/L.sub.p Ratio of beam length to 1-10 1-100
beam span
[0086] As previously described, the present invention provides for
designing spring structures with specific properties, such as
specific spring rates, by contouring the beam shape. FIGS. 6-10
show exemplary shapes for contoured beams according to the
invention. An exemplary U-shaped beam is shown in FIG. 6. U-shaped
beam 60 has a cross-section 50B as shown in FIG. 5B. Similarly to
the V-shaped cross-section shown in FIG. 5A, the U-shaped
cross-section 50B has a substantially higher area moment of inertia
relative to a beam with a flat rectangular cross-section 50C, shown
in FIG. 5C. The U-shaped cross-section avoids the notch in the base
of the V-shape, which may cause undesirable stress concentration.
However, the choice of a U-shaped cross-section relative to other
sectional shapes depends mainly on considerations other than spring
rate or other spring performance parameters. For example,
manufacturability is an important consideration. Depending on the
preferred manufacturing method, a particular shape, such as the
U-shape, may be less costly to manufacture than other shapes.
[0087] FIGS. 7A and 8 show various ribbed beams. FIG. 7A shows a
contoured beam 64 having a rib 63 disposed above the surface of the
beam. In beam 64, rib 63 runs the entire length of the beam and
base 28. As shown in FIG. 7B, beam 64 is also preferably contoured
lengthwise along a compound curve 34, as previously described. Rib
63 preferably follows the contour of curve 34, although it may be
tapered in the z-direction to approach the upper surface of beam 64
at its tip. FIG. 8 shows a contoured beam 62 having a rib 63
disposed beneath the beam. Rib 63 is terminated at an edge of base
28. Ribbed beams may be somewhat more costly to fabricate than
V-shaped or U-shaped beams, because of their more intricate shapes,
but can provide certain advantages. One advantage is that ribs can
be placed across the width of a beam wherever greater stiffness is
desired. For example, as shown in FIG. 5D, ribs may be placed on
the edges of a beam to stiffen the beam against torsion. This may
be desirable where a beam having an offset from a line between its
base and its tip, such as the C-shaped beam shown in FIGS. 13A and
13B, is desired. Offset beams are subject to torsion in portions of
the beam, and properly placed ribs can stiffen the offset beam
against torsion, where desired. Another advantage is that a rib can
be extended into the base of a microelectronic spring structure, as
shown in FIGS. 7A and 7B. This prevents stress concentration and
beam failure at the juncture 27 between base 28 and beam 30, that
can result from abrupt changes in cross-sectional shape along a
beam. Ribs 63 according to the present invention are preferably
comprised of a folded portion of the beam 30, as shown in each of
the foregoing FIGS. 5D and 7A-8. The folded portion is preferably
configured so that no portion of the rib overlaps the beam, when
viewed from above the beam looking towards the substrate.
Non-overlapping, folded ribs may be formed by deposition of a layer
of material over a molded form. Such ribs tend to have a hollow
interior, as is evident in each of the foregoing FIGS. 5D and 7A-8.
A folded rib thus has the same thickness "t" as the beam 30, and
forms an integral portion of the beam. The cross-section of ribs
may be rectangular, triangular, cylindrical, or another shape, and
may taper in width or height along the length of the spring
beam.
[0088] Contouring according to the present invention can be used
for a completely different purpose, apart from providing an
increased area moment of inertia for stiffening the spring, as
described above. A spring may also be lengthwise contoured to
decrease its "footprint," i.e., the amount of projected area the
spring occupies on substrate 23. FIG. 9A shows a corrugated spring
structure 66 that is contoured to decrease its footprint. An
exemplary shape of the corrugations is shown in edge view in FIG.
9B. It should be apparent that the corrugations, like ribs, are
readily formed by depositing a layer of material over a molded
form, according to the method disclosed above. It should be further
apparent that the ratio of the length "L" of the beam 30 to its
projected length "L.sub.p" on substrate 23 is increased by the
corrugations. For a cantilevered beam springs having the same ratio
of thickness to length (i.e., "the same "t/L"), the elastic range
is directly proportional to the spring length "L." Accordingly, the
corrugations provide a greater elastic range "c" and a higher
elastic deflection ratio ("c/h") than an uncorrugated beam of
equivalent projected length and z-extension, so long as the
thickness of the spring is increased in proportion to the increase
in spring length.
[0089] A further advantage of a corrugated spring is that the
bending moment experienced by the spring base for a given force
applied at the spring tip is reduced proportionally to the
reduction of projected length "L.sub.p." Reducing the bending
moment at the base reduces the spring base area required to achieve
adequate adhesion of the spring structure to the substrate. A
reduced base area, in turn, further reduces the footprint of the
spring structure. Additionally, corrugations provide resiliency in
the horizontal plane, parallel to the substrate (i.e., in the "x-y
plane"). That is, corrugated springs can be made more mechanically
compliant in the x-y plane than straight and convex designs. This
is advantageous where the spring tip will be fixed in place (e.g.,
if the spring tip must be soldered down to the contact pad). The
resiliency in the x-y plane compensates for thermal expansion
differentials, misalignment, vibration and other stresses between
connected components, thereby increasing the reliability of the
connection.
[0090] Different types of contours may be combined on a single
spring. For example, a combination of contouring along a beam
length and across its width is described above. Similarly, a
corrugated beam may be provided with other contoured features, such
as the central rib 69 shown on the corrugated beam 68 in FIG. 10,
provided to stiffen the spring structure. Alternatively, or in
addition, a corrugated beam may be provided with a further
lengthwise curvature, such as the compound curve 34 shown in FIGS.
1B and 7B. A corrugated beam may also be contoured across its
width, for example, to provide a V-shaped cross-section as shown in
FIG. 5A, or a U-shaped cross-section as shown in FIG. 5B.
[0091] As previously described, for many applications it is
advantageous to taper the beam of a microelectronic spring
structure across its width, thereby creating a triangular shape in
plan view. Triangular beam shapes are more structurally efficient,
although less efficient in conducting electrical current, than
untapered, rectangular beam shapes. Similar considerations apply to
beams that are tapered in thickness, from a maximum thickness at
the base of a spring beam to a minimum thickness at the beam tip.
Tapering the thickness provides a proportionally much greater
stiffening effect than tapering the width, because the area moment
of inertia "I" of a cantilevered beam section is proportional to
the cube of the section thickness, but is only linearly related to
the section width. However, on a lithographic type, it is more
difficult to taper the beam thickness than it is to taper the beam
width. Typical lithographic processes for depositing beam material,
such as electroplating and sputtering, tend to deposit material in
layers of essentially uniform thickness. In comparison, the width
of a lithographic structure is easily controlled using methods
known in the art, such as masking.
[0092] The present invention provides beams of tapered thickness by
providing a plurality of narrowly spaced ribs that taper from a
relatively high profile at the base, to a relatively narrow profile
at the tip of the beam. Such beams are of essentially of uniform
thickness, and so can be formed by conventional techniques for
material deposition, such as electroplating. Because of the large
number of ribs, the beam performs like a beam with a tapered
thickness. An exemplary spring structure with a plurality of
closely spaced ribs is shown in FIGS. 11A-11C. Spring structure 75
is provided with a plurality of ribs 63 extending into base 28,
similarly to the spring structure shown in FIG. 7A. Extending the
ribs into base 28 stiffens the structure 75 at juncture 27, but
reduces adhesion of base 28 to substrate 23. In the alternative,
the ribs may be terminated at the juncture 27 between the beam 30
and base 28, similar to the structure shown in FIG. 8. This
configuration maximizes electrical contact with and adhesion to the
substrate 23, but results in stress concentration between beam 30
and base 28 at juncture 27. In the alternative, the ribs may be
extended partially into the base, combining the benefits of both
approaches. It is evident that such as structure may be formed on a
lithographic type by depositing a material in a contoured, open
mold, as previously described.
[0093] As shown in FIGS. 11B and 11C, ribs 63 provide an apparent
beam thickness "a" equal to the height of the ribs. The apparent
beam thickness "a" provides an effective thickness depending on the
distance "I" between each rib and the material thickness "t." At a
first cross-section taken near base 28, shown in FIG. 11B, ribs 63
are relatively high, but not as closely spaced are near the tip 26.
As exemplified by a second cross-section taken closer to the tip
26, shown in FIG. 11C, the ribs are more closely spaced as they
converge near the tip. The material thickness "t" is essentially
uniform throughout the length of the beam. In alternative
embodiments, the ribs do not converge. Instead, if the beam 30 is
tapered, the number of ribs across the width of the beam 30
decreases towards the tip.
[0094] Contouring may be additionally provided in the x-y plane, to
create non-linear, beams that have a portion offset from a straight
line between the spring base and tip. Offset structures are useful
for forming low base moment, torsional beams. An exemplary
non-linear offset spring structure for reducing the bending moment
on the base is illustrated in FIGS. 12A and 12B. Beam 30 is split
into two arms which are routed back towards and over the base 28.
Tip 26 is preferably disposed over base 28 when viewed in plan view
as shown in FIG. 12B. To allow for forming of the entire structure
22 in a deposition process over a sacrificial mold, base 28 is
optionally provided with a cut-out 67 to provide space for
formation of tip 26. In the alternative, no cut-out 67 is formed in
base 28, and tip 26 is not disposed directly over the base 28, but
instead is disposed closely over it.
[0095] The split-arm, non-linear structure shown in FIGS. 12A and
12B provides several advantages. The non-linear configuration
allows for positioning the tip 26 directly over base 28 (or nearly
so) and thereby greatly reduces the bending moment imposed on base
28 by a vertical contact force on the tip 26. Because of the
reduction or elimination of bending moment, the area of base 28 can
be reduced in size without fear of causing the spring structure to
become unattached from substrate 23. Another advantage in that the
effective length of the spring structure is increased for a given
footprint, as previously described with respect to corrugated beam
springs. Thus, elastic range "c" of the spring is increased
compared to a shorter spring with the same footprint, provided the
ratio t/L is the same. Additionally, beam 30 may be designed for
torsion in the transition area 71 where the split arms meet the
central beam at a right angle, providing further opportunities for
fine-tuning the performance characteristics of the spring structure
by building appropriate sectional properties in the transition
area. As is evident from FIG. 12B, the entire spring structure may
be formed by deposition of spring material in an open mold. It
should further be apparent that the x-y contoured shape may be
combined with the other types of contoured shapes, such as ribs,
thickness tapers, and contoured cross-sections.
[0096] Another example of a non-linear, offset contoured spring is
shown in FIGS. 13A and 13B. This example is C-shaped in plan view,
as shown in FIG. 13B, and has a single beam 30 extending back
towards and above base 28. Unlike the spring structure shown in
FIG. 12A, this single-beam structure has its tip 26 disposed a
distance L.sub.p from the centroid of base 28. Thus, this design
greatly reduces, but does not eliminate, the bending moment on base
28. Of course, if desired, the tip could be routed over the base in
a manner similar to the non-linear spring shown in FIG. 12A.
[0097] Yet another example of a non-linear, offset contoured spring
is shown in FIGS. 14A and 14B, showing a spring structure which is
serpentine in plan view. In this example, the tip 26 is not routed
back towards the base 28, although a serpentine spring could be so
routed, if desired. However, it is evident from FIG. 14B that the
ratio of the beam span L.sub.p to the integrated beam length L is
substantially less than one, so the moment on base 28 is
correspondingly reduced. A serpentine spring has the advantage of
providing a relatively long integrated beam length in a compact
footprint. Also, in a serpentine spring, torsional stress is
distributed among the numerous bends of the serpentine structure,
rather than being concentrated in a single bend. It should be
understood from the foregoing examples that a great variety of
different offset, x-y contoured shapes are possible without
departing from the scope of the present invention.
[0098] FIGS. 15A and 15B show alternative embodiments of the
invention for making contact between a single contact pad on a
first device and between two or more contact pads on a second
device. FIG. 15A shows a forked, bi-based structure 88, and FIG.
15B shows a forked, dual-tipped structure 90. Spring contacts 88
and 90 are both provided with dual beams 30A and 30B. In bi-based
structure 88, beams 30A and 30B are joined at tip 26, and lead to
respective ones of separate bases 28A and 28B. In dual-tipped
structure 90, beams 30A and 30B diverge from base 28 to respective
ones of the dual tips 26A and 26B. As exemplified by both of
structures 88 and 90, the dimensions (length and width) of the
plural beams in a forked contact structure are independently
controllable. For example, first beam 30A is shorter and wider than
second beam 30B in both structures 88 and 90. It will be apparent
that forked structures having any number of interconnected and
independent beams between plural bases and tips may readily by
formed using the lithographic processes described and referenced
herein.
[0099] Multiple beams, such as dual beams 30A and 30B, can be used
to tune interconnect system frequency response in the same way as a
bond wire. Independent control of the dimensions of each beam in a
forked structure, and of the geometric relationship between the
beams provides control of the inductance of the each beam, and of
their mutual inductance. Fabrication of such structures using a
lithographic process permits control of inductance with a high
degree of precision. Thus, desired characteristics of the frequency
response of an interconnect system, such as passband width or
flatness, can be accurately tuned using structures such as
structures 88 and 90.
[0100] Microelectronic spring structures according to the present
invention may be used in various modes for making electrical
connections between electronic components. In one embodiment, a
spring structure is used to make a reversible connection between
its tip and a contact pad of another electronic component, such as
a printed circuit board (PCB). In this mode, the contact structure
is preferably not deformed past its elastic range, so it may be
repeatedly reused in its original configuration. As known in the
art, conducting liquids may also be used for making or enhancing an
electrical connection by depositing them on one or both of the
spring tip and mating contact pad. Suitable liquids might include
low-melting-point elemental metals such as gallium or mercury,
metal alloys such as NewMerc.TM. (a low-vapor-pressure mercury
replacement material), and oil or grease filled with conductive
particles such as carbon black. In another embodiment, a contact
structure according to the present invention is used to more
permanently join, such as with solder, its tip to a contact pad of
another electronic component. In this case, the contact structures
should react compliantly in the "x-axis" and/or "y-axis", to
accommodate differences in thermal expansion coefficients between
two electronic components.
[0101] In the case of contact structure used in a resilient,
reversible mode, electrical performance can be enhanced by adding
specialized tip components. Co-pending, commonly assigned U.S.
patent application Ser. No. 08/819,464, entitled "CONTACT TIP
STRUCTURES FOR MICROELECTRONIC INTERCONNECTION ELEMENTS AND METHODS
OF MAKING SAME", and corresponding PCT application S.N.
PCT/US97/08606, published Nov. 20, 1997 as WO97/43653, both of
which are incorporated herein by reference, describe a method for
defining a tip structure on a sacrificial substrate and
transferring that structure to an microelectronic spring structure.
It should be apparent that such methods may be adapted for use with
the present invention. Examples of contact tips for use with
microelectronic spring structures according to the present
invention are provided in FIGS. 16A-16E.
[0102] FIG. 16A shows an integral pointed tip structure. FIG. 16B
shows a spherical integral tip, which is preferable for making more
permanent connections. Various shapes of integral tips may be
provided on a microelectronic spring structure according to the
present invention, by providing a complementary mold shape in the
layer of sacrificial mold material. For example, a spherical tip
may be formed by providing a hemispherical shape in the sacrificial
layer at an appropriate position. Integral tips, whether pointed,
spherical, or some other shape, provide the advantages of
simplicity and lower cost, because they can be integrally formed
with the beam of the spring structure, thus avoiding the need for
additional process steps. However, they have the disadvantage of
limiting the choice of materials and shapes for the conductive tip.
The resilient spring material (such as a nickel alloy) and/or a
conductive metal (such as gold) plated thereon is usually not the
best choice for tip material. For many applications, a harder and
sharper structure is desired, for cutting through layers of oxide
and contamination on the surface of a contact pad. For some
applications, such as for contacting C4 solder balls, a pyramidal
tip of a harder material is better suited, as shown in FIG. 16C.
Pyramidal tips are preferably formed in an independent operation,
and adhered to the tip end of a beam, as further described in the
previously referenced, co-pending U.S. patent application Ser. No.
08/819,464. Suitable material for a pyramidal tip includes, for
example, palladium-cobalt alloy, tungsten, nickel alloys, diamond,
and diamond dust composites. The pyramidal tip is preferably plated
with a conducting layer, such as soft gold, after being adhered to
the tip end of a beam. Multiple pyramidal tips may be used, and
geometric shapes other than pyramids may be used, so long as
sufficiently abrasive to cut through layers of oxide and
contamination on the surface of a contact pad. For example, a
plurality of randomly-shaped abrasive particles, such as diamond
dust with an average particle diameter of less than about 10 .mu.m,
may be adhered to a spherical tip about 100 .mu.m in diameter, and
plated with a conducting metal.
[0103] In an embodiment of the invention, different tip structures
are combined for the purpose of redundancy, or to serve in
multi-purpose applications. For example, a pointed tip, a pyramidal
tip, and a spherical tip may be combined in parallel as in the
parallel combination tip 84 shown in FIG. 16D. Perhaps more
typically, a similar parallel tip structure may be used, wherein
each of the parallel tips is the same type, and the primary purpose
for the redundant tips is increased reliability. In a combination
tip structure the contact structure is also improved for use with a
wider range of applications. For example, the pyramidal tip 82
shown in FIG. 16D could be used to make repeatable connections
during a testing phase of an integrated circuit, and spherical tip
80 would be used to make a long term connection for the same
circuit, such as to a printed circuit board of an electronic
component. Pointed tip 78 could serve as a general purpose,
redundant tip, in case one of tips 80 or 82 is damaged. In the
alternative, one of the tips, such as tip 78, could be omitted.
Similar benefits may be achieved by combining different types of
tips in series, as in the serial combination tip shown in FIG. 16E.
Serial combination tip 86 comprises a pyramidal tip 82 mounted to
an integrally formed spherical tip 80. Thus, the same combination
tip 86 is suited for both repeated, short term connections with
pyramidal tip 82, and for long term connections, using spherical
tip 80.
[0104] It should be understood that it is preferable to select
from, and combine in various ways, the structures described and
exemplified above, depending on the specific requirements of a
particular microelectronics contact application. For example, a
single substrate may be provided with different types of springs,
for example, relatively large structures for providing power and
ground connections, smaller structures for transmission of digital
signals, and forked structures for tuning. Additionally, an
application may have requirements that can be satisfied by two or
more shapes or structures according to the present invention, of
which no single shape is inherently superior. Furthermore,
structures according the present invention are not limited to use
on integrated circuit substrates, but may also be used in other
applications, such as sockets or connectors. In general, the shape
contouring provided by the present invention provides versatile
design solutions for mass-producible spring contacts at a scale far
smaller than has heretofore been achieved, thereby opening up a
wide range of potential applications for such devices.
[0105] Having thus described a preferred embodiment of a
microelectronic spring structure, it should be apparent to those
skilled in the art that certain advantages of the within system
have been achieved. It should also be appreciated that various
modifications, adaptations, and alternative embodiments thereof may
be made within the scope and spirit of the present invention. For
example, a microelectronic spring structure for making electrical
connections to semiconductor devices has been illustrated, but it
should be apparent that the inventive concepts described above
would be equally applicable to a lithographic type spring structure
used as a non-electrically-conducting mechanical spring. For
further example, specific contoured shapes have been disclosed
herein, but it should be apparent that the inventive concepts
described above would be equally applicable to shape variations
made thereon, and to alternative contoured shapes suitable for
lithographic type, integrally formed microelectronic spring
structures. The invention is further defined by the following
claims.
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